Experimental investigation on effects of compression ratio and exhaust gas recirculation on backfire, performance and emission characteristics in a hydrogen fuelled spark ignition engine

• Improvement in power output and thermal efficiency with increase in compression ratio of hydrogen fuelled SI engine.
• The compression ratio and equivalence ratio are limited by knocking and safe engine operation point of view.
• Exhaust gas recirculation is more suitable strategy than adjustment of spark timing for NOx emission reduction.
• Backfire limit start of gas injection decreased with increase in compression ratio.

The experimental study was conducted on single cylinder, forced air cooled hydrogen fuelled spark ignition (SI) generator set, which was converted from gasoline fuelled generator set with rated power 2.1 kVA at 3000 rpm. The study was carried out at various compression ratios (4.5:1, 6.5:1 (base) and 7.2:1), spark timings (2–20 °CA before top dead centre (bTDC)) and exhaust gas recirculation (EGR) up to 25% by Volume. Furthermore, the experimental tests were conducted on the engine with varied start of gas injection (SOI) at various compression ratios in order to find the backfire limiting start of injection (BFL-SOI). The results indicated that engine operation at higher compression ratio improved the brake thermal efficiency and reduced the backfire occurrence as residual gas fraction decreased with increased compression ratio. However, NOx emission increased with increased compression ratio. In order to reduce the NOx emission at source level, the engine was operated with retarded spark timings and different EGR percentage. The relative NOx emission decreased up to 10% with the spark time retarding of 2° CA bTDC from maximum brake torque (MBT) whereas it decreased about 57% with 25% by volume EGR. The delay in gas injection could reduce the chance of backfire occurrence and the BFL-SOI decreased with increased compression ratio. A notable point emerged from this study is that in hydrogen fuelled spark ignition engine the spark time retarding is not a suitable strategy for NOx emission reduction whereas the EGR at the optimum level (20%) is a better strategy that could reduce the NOx emission up to 50% as compared to base hydrogen engine without EGR.